You only need that bootloader file if you plan to burn a new one to the chip. Since the chips you bought already contain the bootloader, you shouldn't have to do anything other than upload thru the existing bootloader by connecting a comm port adapter to pins 2 and 3 on the chip, connect the crystal AND the 22pf caps that you need with them.

Closing the IDE, adding that entry to the boards.txt file and then starting the IDE should provide the entry you need to select to upload. You will also need to select the correct serial port to go with it.

I think you have all you need to make it work unless you wipe out the bootloader on the chip. You will then need to modify the Makefile in the optiboot folder to build a bootloader with the appropriate options and burn it using ArduinoISP or something like that.

You should be able to breadboard up one of your new chips and then push the reset button. You should see an LED connected to DP13 blink three times. This means the chip and bootloader is running.

If you use another board running ArduinoISP to flash one of your pre-bootloaded chips, the flash process will erase the chip, including the bootloader. You will then be forced into using the "burn bootloader" feature of the IDE. You would have to use a working board running ArduinoISP to do so. You will also need a bootloader out there that matches the name used in the boards.txt file. I assume these chips came configured to use a 16MHz crystal, is that correct?

That's one of the weird things about all this that you have to understand. New, blank chips (no bootloader, nothing) need to have the fuses set. This happens when you install a bootloader. Seems like ArduinoISP should be able to do this, but apparently it can't. I don't know if a "regular" external programmer can do it or not? I don't see what is so special about the "burn bootloader" process that sets the fuse and lock bits, that can't be done ordinarily when you upload.